Liquid crystal composition containing a pleochroic azo type compound

ABSTRACT

A liquid crystal composition suitable for a guest-host liquid crystal device and the composition comprises a solution of a liquid crystal substance and a pleochroic dye which comprises at least one azo type compound having the formula (I), ##STR1## wherein R 1  represents a hydrogen atom, an alkyl group or an alkoxy group each of which may be substituted by an alkoxy or an aryl group; a cyclohexyl group which may be substituted by an alkyl group; or --COOR 3  (wherein R 3  represents an alkyl group which may be substituted by an alkoxy group, a cyclohexyl group which may be substituted by an alkyl group, or an aryl group which may be substituted by an alkoxy or an alkyl group); 
     R 2  represents ##STR2## where R 4  and R 5  each represents an alkyl group which may be substituted by an alkoxy or an aryl group; or R 4  and R 5  may be connected to each other to form a ring; 
     Z 1  and Z 2  each represents a hydrogen atom, a halogen atom, and alkyl group or an alkoxy group, or Z 1  and Z 2  may be connected to each other to form a carbocyclic ring or a nitrogenous heterocyclic ring; 
     m is 1 or 2; and 
     n is 0 or 1.

FIELD OF THE INVENTION

This invention relates to a guest-host liquid crystal compositioncontaining a pleochroic azo type compound.

BACKGROUND OF THE INVENTION

There has so far been known to effect color display in utilization ofthe guest-host effect between a dye and liquid crystal by interposingbetween a pair of mutually opposing electrodes a liquid crystalcomposition prepared by dissolving a dye into liquid crystal.

The dye to be used for such liquid crystal composition is required tohave, among other things, high dichroic ratio, high solubility to theliquid crystal, and other properties.

However, these conventionally known azo type compounds did not alwayssatisfy the properties which are required of the dye to be used in theliquid crystal composition as mentioned above.

SUMMARY OF THE INVENTION

The object of this invention is, therefore to provide guest-host liquidcrystal compositions comprising an azo type compound satisfying theabove-described factors.

With a view to attaining such objective, the present inventors conductedrepeated studies and researches, as the result of which they have cometo a finding that, the azo type compounds which are represented by thefollowing formula (I), exhibit high dichroic ratio and good solubilityto liquid crystal, and that liquid crystal compositions containing suchdye perform satisfactory color display. Based on this finding, they havearrived at the present invention.

Therefore, this invention has its gist in providing liquid crystalcompositions containing azo type compounds, which are represented by thefollowing formula (I), ##STR3## wherein R¹ represents a hydrogen atom,an alkyl group which may be substituted by an alkoxy or aryl group, analkoxy group which may be substituted by an alkoxy or aryl group, acyclohexyl group which may be substituted by an alkyl group, or R³ OOC--(where R³ represents an alkyl group which may be substituted by analkoxy group, a cyclohexyl group which may be substituted by an alkylgroup, or an aryl group which may be substituted by an alkoxy or alkylgroup); R₂ represents ##STR4## (where R⁴ and R⁵ each represents an alkylgroup which may be substituted by an alkoxy or aryl group, and R⁴ and R⁵optionally connect each other to form a ring); Z¹ and Z² each representsa hydrogen atom, a halogen atom, an alkyl group or an alkoxy group andmay connect each other to form a carbocyclic ring or a nitrogenousheterocyclic ring; m represents 1 or 2; and n represents 0 or 1).

DETAILED DESCRIPTION OF THE INVENTION

Then the invention is explained in detail.

First, the azo type dyes shown by formula (I) described above areexplained.

In the azo type compounds represented by the general formula (I) in thepresent invention, R¹ may be selected from among the following: ahydrogen atom; a linear or branched alkyl group (preferably an alkylgroup having 1 to 8 carbon atoms); a linear or branched alkoxy group(preferably an alkoxy group having 1 to 8 carbon atoms); an alkyl group(preferably an alkyl group having 1 to 8 carbon atoms) and an alkoxygroup (preferably an alkoxy group having 1 to 8 carbon atoms) which aresubstituted by an alkoxy group (preferably an alkoxy group having 1 to 8carbon atoms); an alkyl group (preferably an alkyl group having 1 to 3carbon atoms) and an alkoxy group (preferably an alkyl group having 1 to3 carbon atoms) which are substituted by an aryl group such as a phenylgroup which may be substituted by an alkyl group having 1 to 8 carbonatoms, an alkoxy group having 1 to 8 carbon atoms or a halogen atom suchas a chlorine atom; a cyclohexyl group which may be substituted by analkyl group having 1 to 8 carbon atoms; and --COOR³ [where R³ is analkyl group (preferably an alkyl group having 1 to 8 carbon atoms) whichmay be substituted by an alkoxy group (preferably an alkoxy group having1 to 8 carbon atoms), a cyclohexyl group which may be substituted by analkyl group (preferably an alkyl group having 1 to 8 carbon atoms), oran aryl group such as a phenyl or naphthyl group which may besubstituted by an alkyl group (preferably an alkyl group having 1 to 8carbon atoms) or an alkoxy group (preferably an alkoxy group having 1 to8 carbon atoms)].

In the general formula (I), substituents R⁴ and R⁵ each represents analkyl group which may be substituted by an alkoxy or aryl group and eachof them may be exemplified by the following: an alkyl group having 1 to8 carbon atoms; an alkyl group having 1 to 3 carbon atoms which issubstituted by an alkoxy group having 1 to 8 carbon atoms; and an alkylgroup having 1 to 3 carbon atoms which is substituted by ##STR5## (whereR⁶ is a hydrogen atom, an alkyl group having 1 to 8 carbon atoms, analkoxy group, or a halogen atom such as a chlorine atom). If desired, R⁴and R⁵ may connect each other to form a nitrogenous heterocyclic ringsuch as a piperidine ring, a pyrrolidine ring, a piperazine ring, or amorpholine ring.

In formula (I), substituents Z¹ and Z² each represents a hydrogen atom,an alkyl group such as an alkyl group having 1 to 3 carbon atoms, analkoxy group such as an alkoxy group having 1 to 3 carbon atoms, or ahalogen atom such as a chlorine atom; Z¹ and Z² may connect each otherto form a carbocyclic ring such as a benzene ring, a cyclohexane ring ora nitrogenous heterocyclic ring such as a pyridine ring.

The azo type compounds suitable for use in the present invention may beillustrated by:

(i) azo type compounds of the following general formula (II) where n=1in formula (I): ##STR6## [where R¹, R², Z¹, Z² and m are the same asdefined in formula (I)]; and

(ii) azo type compounds of the following general formula (III) where n=0in formula (I): ##STR7## [where R¹, R², Z¹, Z² and m are the same asdefined in formula (I)].

Azo type compounds that are particularly suitable for use in the liquidcrystal composition of the present invention are described hereinafter.

Among the azo type compounds represented by the general formula (I),particularly preferred are those which are represented by the followinggeneral formula (IV): ##STR8## wherein R¹ has the same meaning asdefined in formula (I) and is preferably selected from among: (i) analkyl group optionally substituted by an alkoxy or aryl group; (ii) analkoxy group optionally substituted by an alkoxy or aryl group; (iii) acyclohexyl group optionally substituted by an alkyl group. Preferably,R¹ is (i) or (iii), the former being an alkyl group which may besubstituted by an aryl group. An alkyl group or a 4-alkylcyclohexylgroup is particularly preferred as R¹.

While R² has the same meaning as defined in formula (I), it ispreferably ##STR9## (where R⁴ and R⁵ each represents an alkyl groupwhich may be represented by an alkoxy or aryl group). More preferably,R² represents ##STR10## (where R⁷ is a hydrogen atom or an alkyl group,and R⁶ is a hydrogen atom, an alkyl group or an alkoxy group) or##STR11## (where R⁴ and R⁵ each represents an alkyl group which may besubstituted by an alkoxy group), with R⁴ and R⁵ being preferably analkyl group.

More preferably, R² is represented by ##STR12## (where R⁶ represents analkyl group or an alkoxy group) or ##STR13## the former beingparticularly preferred.

In formula (IV), ##STR14##

A particularly preferred case is where each of ##STR15## In theabove-illustrated examples of ##STR16## Z³ represents a hydrogen atom oran alkyl group, with a hydrogen atom or a methyl group being preferred.

In formula (IV), m represents a number of 1 or 2, and n represents anumber of 0 or 1. When m is 1, R¹ preferably represents an alkyl groupor a 4-alkylcyclohexyl group, and when m is 2, R¹ preferably representsan alkyl group. From a practical viewpoint, compounds where n=1 arepreferred over those where n=0.

The azo type compounds represented by the general formula (II) may beprepared as follows in accordance with the method described in JapanesePatent Laid-Open No. 38756/83:

diazotizing a monoazo compound represented by the general formula (V):##STR17## wherein R¹, m, Z¹ and Z² have the same meanings as defined inthe general formula (II); and coupling the resulting diazo compound withan aniline represented by the general formula (VI): ##STR18## where R²and ##STR19## have the same meanings as defined in the general formula(II).

The azo type compounds represented by the general formula (III) may beprepared by the same procedures as above except that the compound offormula (V) is replaced with the compound represented by the followinggeneral formula (VII): ##STR20## where R¹, Z¹, Z² and m have the samemeanings as defined in formula (III).

The liquid crystal composition of the present invention contains thecompound of formula (I) in an amount ranging from 0.01 to 10 wt%,preferably from 0.05 to 3 wt%, of the liquid crystal substance.

The liquid crystal composition of the present invention is a compoundthat is to be used in guest-host type liquid-crystal displays. The term"guest-host type" means the incorporation of a dichroic dye (guest) in aliquid-crystal host. Examples of the guest-host type display include atwisted nematic type (TN) display method, a heat writing (HW) displaymethod, and a guest-host display method employing or not employing apolarizing film.

While various liquid crystals may be used in the liquid crystalcomposition of the present invention, typical examples include nematicliquid crystals, smectic liquid crystals (e.g. ferroelectric liquidcrystals such as smectic A and chiral smectic C structures), cholestericliquid crystals, and polymer liquid crystals.

The nematic liquid crystal to be used for the liquid crystal compositionaccording to the present invention may be selected from fairly broadvarieties, provided that they are able to exhibit the nematic statewithin a range of their operating temperatures. It may also be possiblethat such a nematic liquid crystal is caused to assume the cholestericstate by addition thereto of an optically active substance to bedescribed later. As the nematic liquid crystal, there may be exemplifiedthose substances as shown in No. 1 to No. 3 of the following Table 1 ortheir derivatives.

                                      TABLE 1                                     __________________________________________________________________________    No. Type of Liquid Crystal                                                                      Example                                                     __________________________________________________________________________    1   Cyclohexylcyclohexane type                                                                   ##STR21##                                                  2   Phenylcyclohexane type                                                                       ##STR22##                                                  3   Biphenyl type                                                                                ##STR23##                                                  4   Terphenyl type                                                                               ##STR24##                                                  5   Cyclohexylcyclohexanate type                                                                 ##STR25##                                                  6   Phenylcyclohexyl- carboxylate type                                                           ##STR26##                                                  7   Ester type                                                                                   ##STR27##                                                  8   Diester type                                                                                 ##STR28##                                                                     ##STR29##                                                  9   Biphenyl cyclohexyl carboxylate type                                                         ##STR30##                                                  10  Biphenyl ester type                                                                          ##STR31##                                                                     ##STR32##                                                  11  Thioester type                                                                               ##STR33##                                                  12  Schiff type                                                                                  ##STR34##                                                                     ##STR35##                                                  13  Pyrimidine type                                                                              ##STR36##                                                                     ##STR37##                                                                     ##STR38##                                                                     ##STR39##                                                  14  Dioxane type                                                                                 ##STR40##                                                  15  Cyclohexyl methylether type                                                                  ##STR41##                                                                     ##STR42##                                                  16  Cinnamonitrile type                                                                          ##STR43##                                                  17  Ethane type                                                                                  ##STR44##                                                                     ##STR45##                                                                     ##STR46##                                                  18  Dicyanophenyl ester type                                                                     ##STR47##                                                                     ##STR48##                                                                     ##STR49##                                                                     ##STR50##                                                  __________________________________________________________________________

In the above Table 1, R^(a) denotes an alkyl group or an alkoxy group;R^(b) denotes an alkyl group; and X represents an alkyl group, an alkoxygroup, a nitro group, a cyano group, or a halogen.

While the host liquid crystal substance to be used for the liquidcrystal composition according to the present invention may be selectedfrom any of those liquid crystals shown in Table 1 above or theirmixtures, the liquid crystal produced and marketed by Merck & Co. undera tradename of "ZLI-1132"; the liquid crystal produced and marketed byMerck & Co. under a tradename of "ZLI-1565"; and the liquid crystalproduced and marketed by British Drug House & Co. under a tradename of"E-7", are recommended as useful.

As the optically active substances to be used in the liquid crystalcomposition according to the present invention, there may beexemplified, chiral nematic compounds, for example, those compoundshaving optically active groups such as a 2-methylbutyl group, a3-methylbutoxy group, a 3-methylpentyl group, a 3-methylpentoxy group, a4-methylhexyl group, a 4-methylhexyloxy group, etc. introduced into thenematic liquid crystals; alcohol derivatives disclosed in JapaneseUnexamined Patent Publication No. 45546/76 such as λ-menthol andd-borneol; ketone derivatives such as d-camphor and 3-methylcyclohexane; carboxylic acid derivatives such as d-citronellic acid andλ-camphoric acid; aldehyde derivatives such as d-citronellal; alkenederivatives such as d-linonene; or other optically active substancessuch as amines, amides and nitrile derivatives.

As the elements for performing the liquid crystal display by use of theliquid crystal composition according to the present invention, there maybe used known liquid crystal display elements. In more detail,transparent electrodes of any arbitrary patterns are formed on each oftwo sheets of glass substrates, at least one of which is transparent,and then these two sheets of glass substrates are arranged in parallelrelationship such that the surface sides of the glass substrates, onwhich the electrodes have been formed, may be mutually opposed withappropriate spacers being interposed between them to construct theliquid crystal display element to be used. In this case, a space gap forthe element is determined by the spacers. A preferred range of the spacegap for the liquid crystal display element may be from 1 to 100 μm, ormore preferably from 2 to 50 μm from the practical standpoint.

The azo type compounds for use in the present invention present violetto blue colors and feature high dichroic ratio and solubility in liquidcrystals. Therefore, the liquid crystal composition of the presentinvention containing such compounds enable high-contrast color displaynot only in the blue range but also in black color when compounded withother pigments. In addition, the liquid crystal composition of thepresent invention is very useful in providing quick-response display.

In the following, the present invention will be explained moreconcretely in reference to practical examples, although the presentinvention is not restricted by these examples.

Here, explaining briefly about the order parameter S for indicating thedichroism, it can be empirically found on any dye from the followingequation. ##EQU1##

In the above equation, A// and A respectively denote light absorbancesof the dye with respect to light beams which have been polarized inparallel with and perpendicular to, respectively, the direction oforientation of the liquid crystal.

Specifically, the order parameter S is a value which indicates a displaycontrast of the guest-host type liquid crystal element, according towhich the nearer this value comes theoretically to 1, the less becomesthe degree of color remnant in the whitened portion, whereby a cleardisplay of high brightness and contrast can be accomplished.

EXAMPLE 1-1

The azo type compound (dye) which is represented by the followingformula: ##STR51## was added to the afore-mentioned mixed liquid crystalZLI-1565 (the tradename of the product of Merck & Co.), and the batchwas heated to a temperature of 70° C. or higher, followed by agitatingthe same when it assumed a state of isotropic liquid, after which themixture was left to cool. These process steps were repeated to welldissolve the above-mentioned compound (dye) into the liquid crystal.

The thus prepared liquid crystal composition according to the presentinvention was sealed into a space gap of 10 μm of an element composed ofa pair of upper and lower glass substrates, each having a transparentelectrode and been subjected to treatment, wherein the surface side ofthe glass substrate in contact with the liquid crystal was coated with apolyamide type resin, and cured, after which the resin was rubbed tohave the homogeneous orientation. The above-mentioned liquid crystalwithin the element as treated for the above-mentioned homogeneousorientation took that homogeneous orientation when no electric voltagewas applied to it, and the molecules of the dye also took the sameorientation in accordance with the host liquid crystal.

The thus fabricated guest-host element was measured for its absorptionspectrum by use of a light polarized in parallel with the direction oforientation of the liquid crystal molecules, and the light polarizedvertically thereto, from which the maximum absorbing wavelength of thedye to each of these polarized lights were found. In finding out theabsorbance of the dye, there was made corrections on the absorption bythe host liquid crystal and the glass substrates as well as thereflection loss of the element.

Also, using the absorbances A// and A with respect to each of thepolarized lights the values of the order parameter S were calculatedbased on the afore-mentioned equation of: ##EQU2##

The results from the above calculation are shown in No. 1 of Table 2below.

The azo type compound used in Example 1-1 was prepared by the followingprocedures: 5 g of 2-amino-5-(4'-n-butyl)phenylazothienothiazole wasdiazotized with 5.2 g of 43% nitrosyl sulfate at 0° C. in a mixedsolvent consisting of N-methylpyrrolidone (100 ml), acetic acid (50 ml)and propionic acid (50 ml), and then subjected to coupling reaction with2.2 g of N-ethyl-N-methylaniline at 0° C. The resulting azo typecompound had a melting point of 239° to 240° C.

EXAMPLE 1-2

An azo type compound represented by the following formula: ##STR52## wasprocessed as in Example 1-1 to fabricate a liquid crystal composition inguest-host element. The maximum absorption wavelength and orderparameter (S) of the dye compound were measured and calculated as inExample 1-1. The results are shown in Table 2 under No. 2.

The azo type compound used in Example 1-2 was prepared as described inExample 1-1 except that N-ethyl-N-methylaniline was replaced byN-(p-n-pentylbenzyl)-3-methylaniline. This azo type compound had amelting point of 190° to 191° C.

EXAMPLES 1-3 TO 1-33

Azo type compounds (dyes) of the present invention having the structuralformulas shown in Table 2 under Nos. 3 to 33 were prepared and isolatedas in Example 1-1. The so prepared 31 dye compounds were processed as inExample 1-1 to fabricate liquid crystal compositions in guest-hostelement. The maximum absorption wavelengths (λmax) and order parameters(S) of the respective dye compounds were measured and calculated as inExample 1-1. The results are shown in Table 2 under Nos. 3 to 33.

    TABLE 2      ##STR53##              No. R.sup.1     ##STR54##      m      ##STR55##      R.sup.2 λmax(nm) order.paramete(S)                1 (n)H.sub.9      C.sub.4     ##STR56##      1      ##STR57##      ##STR58##      592 0.78   2 (n)H.sub.9      C.sub.4     ##STR59##      1      ##STR60##      ##STR61##      590 0.80      3 H     ##STR62##      1      ##STR63##      ##STR64##      592 0.76   4 (n)H.sub.17      C.sub.8     ##STR65##      1      ##STR66##      ##STR67##      590 0.81      5     ##STR68##      ##STR69##      1      ##STR70##      ##STR71##      592 0.77   6 (n)H.sub.11 C.sub.5      O     ##STR72##      1      ##STR73##      ##STR74##      598 0.79   7 (n)H.sub.15 C.sub.7      O     ##STR75##      1      ##STR76##      ##STR77##      593 0.81   8 (n)H.sub.9      C.sub.4     ##STR78##      1      ##STR79##      ##STR80##      592 0.78   9 (n)H.sub.9      C.sub.4     ##STR81##      2      ##STR82##      ##STR83##      593 0.82  10 (n)H.sub.17      C.sub.8O     ##STR84##      2      ##STR85##      ##STR86##      593 0.82      11     ##STR87##      ##STR88##      1      ##STR89##      ##STR90##      600 0.79      12     ##STR91##      ##STR92##      1      ##STR93##      ##STR94##      595 0.78      13 CH.sub.3     ##STR95##      1      ##STR96##      ##STR97##      590 0.78  14 (n)H.sub.9      C.sub.4     ##STR98##      1      ##STR99##      ##STR100##      588 0.79  15 (n)H.sub.15      C.sub.7     ##STR101##      1      ##STR102##      ##STR103##      592 0.78  16 (n)H.sub.9      C.sub.4     ##STR104##      1      ##STR105##      ##STR106##      588 0.81  17 (n)H.sub.9 C.sub.4 OH.sub.4      C.sub.2O     ##STR107##      1      ##STR108##      ##STR109##      590 0.80      18     ##STR110##      ##STR111##      2      ##STR112##      ##STR113##      595 0.83  19 (n)H.sub.13 C.sub.6      OCH.sub.2     ##STR114##      1      ##STR115##      ##STR116##      590 0.80      20     ##STR117##      ##STR118##      1      ##STR119##      ##STR120##      590 0.81      21     ##STR121##      ##STR122##      1      ##STR123##      ##STR124##      593 0.82      22     ##STR125##      ##STR126##      1      ##STR127##      ##STR128##      590 0.81      23     ##STR129##      ##STR130##      2      ##STR131##      ##STR132##      595 0.82  24 (n)H.sub.11      C.sub.5     ##STR133##      1      ##STR134##      ##STR135##      642 0.78      25     ##STR136##      ##STR137##      1      ##STR138##      ##STR139##      640 0.81  26 (n)H.sub.11      C.sub.5     ##STR140##      2      ##STR141##      ##STR142##      635 0.81      27     ##STR143##      ##STR144##      1      ##STR145##      ##STR146##      635 0.81  28 (n)H.sub.9      C.sub.4     ##STR147##      2      ##STR148##      ##STR149##      610 0.80  29 (n)H.sub.13 C.sub.6      OOC     ##STR150##      2      ##STR151##      ##STR152##      607 0.81      30     ##STR153##      ##STR154##      1      ##STR155##      ##STR156##      610 0.80      31     ##STR157##      ##STR158##      1      ##STR159##      ##STR160##      608 0.81  32 (n)H.sub.15      C.sub.7     ##STR161##      2      ##STR162##      ##STR163##      612 0.81      33     ##STR164##      ##STR165##      1      ##STR166##      ##STR167##      592 0.78     ##STR168##

EXAMPLE 2-1

An azo type compound represented by the following structural formula:##STR169## was processed as in Example 1-1 to fabricate a liquid crystalcomposition in guest-host element. The maximum absorption wavelength andorder parameter (S) of the compound were measured and calculated as inExample 1-1. The results shown in Table 3 under No. 1.

The azo type compound used in Example 2-1 was prepared by the followingprocedures: 4.4 g of2-amino-5-[4"-(4'-n-pentyl)phenylazo]phenylazothienothiazole wasdiazotized with 3.5 g of 43% nitrosyl sulfate at 0° C. in a mixedsolvent consisting of phosphoric acid (200 ml) and acetic acid (40 ml),and then subjected to coupling reaction with 26.5 g ofN-(p-n-pentylbenzyl)-3-methylaniline at 0° C. The resulting azo typecompound had a melting point of 198° to 199° C. Molecular extensioncoefficient measurement in chloroform gave a value of 6.50×10⁴. Thisshows that the azo type compound of the present invention which wasprepared in Example 2-1 had a very great coloring power and needed to beincorporated in only a small amount when preparing a desired liquidcrystal composition.

EXAMPLE 2-2

An azo type compound represented by the formula: ##STR170## wasprocessed as in Example 1-1 to fabricate a liquid crystal composition inguest-host element. The maximum absorption wavelength and orderparameter (S) of the compound were measured and calculated as in Example1-1. The results are shown in Table 3 under No. 2.

The azo type compound used in Example 2-2 was prepared as described inExample 2-1 except that2-amino-5-[4"-(4'-n-pentyl)phenylazo]phenylazothienothiazole wasreplaced by 2-amino-5-(4'-phenylazo)phenylazothienothiazole. This azotype compound had a melting point of 213° to 214° C.

Molecular extinction coefficient measurement of the dye compound inchloroform gave a value of 7.6×10⁴, indicating the great coloring powerof this azo type compound.

EXAMPLES 2-3 to 2-31

Azo type compounds (dyes) of the present invention having the structuralformulas shown in Table 3 under Nos. 3 to 31 were prepared and isolatedas in Example 1-1. The so prepared 29 dye compounds were processed as inExample 1-1 to fabricate liquid crystal compositions in guest-hostelement. The maximum absorption wavelengths (λmax) and order parameters(S) of the respective compounds were measured and calculated as inExample 1-1. The results are shown in Table 3 under Nos. 3 to 31.

    TABLE 3      ##STR171##            No.     ##STR172##      ##STR173##      ##STR174##      λmax(nm) order ·      paramete(S)                                     1      ##STR175##      ##STR176##      ##STR177##      605 0.80      2     ##STR178##      " " 611 0.77      3     ##STR179##      " " 605 0.80      4     ##STR180##      "      ##STR181##      605 0.81      5     ##STR182##      " " 605 0.82      6     ##STR183##      "      ##STR184##      613 0.80      7     ##STR185##      ##STR186##      ##STR187##      615 0.78      8     ##STR188##      ##STR189##      ##STR190##      615 0.80      9     ##STR191##      "      ##STR192##      599 0.81      10     ##STR193##      "      ##STR194##      600 0.83      11     ##STR195##      "      ##STR196##      600 0.83      12     ##STR197##      ##STR198##      ##STR199##      610 0.80      13     ##STR200##      ##STR201##      ##STR202##      610 0.78      14     ##STR203##      ##STR204##      ##STR205##      600 0.80      15     ##STR206##      ##STR207##      ##STR208##      630 0.79      16     ##STR209##      ##STR210##      ##STR211##      615 0.79      17     ##STR212##      ##STR213##      ##STR214##      655 0.80      18     ##STR215##      ##STR216##      ##STR217##      650 0.79      19     ##STR218##      ##STR219##      ##STR220##      612 0.79      20     ##STR221##      ##STR222##      ##STR223##      620 0.82      21     ##STR224##      ##STR225##      ##STR226##      625 0.81      22     ##STR227##      ##STR228##      ##STR229##      615 0.82      23     ##STR230##      ##STR231##      ##STR232##      615 0.82      24     ##STR233##      ##STR234##      ##STR235##      620 0.82      25     ##STR236##      ##STR237##      ##STR238##      622 0.80      26     ##STR239##      ##STR240##      ##STR241##      603 0.80      27     ##STR242##      ##STR243##      ##STR244##      610 0.81      28     ##STR245##      ##STR246##      ##STR247##      609 0.83      29     ##STR248##      ##STR249##      ##STR250##      607 0.83      30     ##STR251##      ##STR252##      ##STR253##      612 0.83      31     ##STR254##      ##STR255##      ##STR256##      610 0.80     (Note:     ##STR257##

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A liquid crystal composition suitable for aguest-host liquid crystal device comprising a solution of a liquidcrystal substance and a pleochroic dye, said pleochroic dye comprisingat least one azo type compound having the formula (I), ##STR258##wherein R¹ represents a hydrogen atom; an alkyl radical having 1 to 3carbon atoms substituted with a phenyl radical which is substituted by amember of the group consisting of an alkyl radical having 1 to 8 carbonatoms, an alkoxy radical having 1 to 8 carbon atoms and, a chlorineatom; an alkyl radical having 1 to 8 carbon atoms which may besubstituted by an alkoxy radical having 1 to 8 carbon atoms; an alkoxyradical having 1 to 3 carbon atoms substituted by a phenyl radical whichis substituted by a member of the group consisting of an alkyl radicalhaving 1 to 8 carbon atoms, an alkoxy radical having 1 to 8 carbon atomsand a chlorine atom; an alkoxy radical having 1 to 8 carbon atoms whichmay be substituted by an alkoxy radical having 1 to 8 carbon atoms; acyclohexyl group which may be substituted by either an alkyl radicalhaving 1 to 8 carbon atoms or --COOR³,wherein R³ represents an alkylradical having 1 to 8 carbon atoms which may be substituted by an alkoxyradical having 1 to 8 carbon atoms; a cyclohexyl radical substituted byan alkyl radical having 1 to 8 carbon atoms; a phenyl radicalsubstituted by an alkoxy radical having 1 to 8 carbon atoms; or a phenylradical substituted by an alkyl radical having 1 to 8 carbon atoms,##STR259## wherein R⁴ represents an alkyl radical having 1 to 3 carbonatoms, optionally substituted by a phenyl radical which is substitutedby a substituent selected from the group consisting of an alkyl radicalhaving 1 to 8 carbon atoms, an alkoxy radical having 1 to 8 carbonatoms, or a chlorine atom; or ##STR260## wherein R^(4') represents analkyl radical having 1 to 8 carbon atoms, and R⁵ represents an alkylradical having 1 to 8 carbon atoms, or an alkyl radical having 1 to 3carbon atoms substituted by a phenyl radical which is substituted by analkyl radical having 1 to 8 carbon atoms, R^(4') and R⁵ being optionallyconnected to form ##STR261## Z¹ and Z² each represent a hydrogen atom, achlorine atom, an alkyl radical having 1 to 3 carbon atoms, or an alkoxyradical having 1 to 3 carbon atoms, or Z¹ and Z² are connected to form afused ring of the formula: ##STR262## m is 1 or 2; and n is 0 or
 1. 2. Aliquid crystal composition as claimed in claim 1 wherein the azo typecompound is represented by the following general formula (II):##STR263## where R¹, R², Z¹, Z² and m are the same as defined in thegeneral formula (I).
 3. A liquid crystal composition as claimed in claim1 wherein the azo type compound is represented by the following generalformula (III): ##STR264## where R¹, R², Z¹, Z² and m are the same asdefined in the general formula (I).
 4. A liquid crystal composition asclaimed in claim 1 wherein the azo type compound is represented by thefollowing general formula (IV): ##STR265## where R¹, R², m and n are thesame as defined in the general formula (I); ##STR266## each represents##STR267## and Z³ represents a hydrogen atom or an alkyl radical.
 5. Aliquid crystal composition as claimed in claim 4, wherein R¹ representsa hydrogen atom; an alkyl radical having 1 to 8 carbon atoms; an alkylradical having 1 to 8 carbon atoms substituted by an alkoxy radicalhaving 1 to 8 carbon atoms; or an alkoxy radical having 1 to 3 carbonatoms substituted by a phenyl radical which is substituted by asubstituent selected from the group consisting of an alkyl radicalhaving 1 to 8 carbon atoms, an alkoxy radical having 1 to 8 carbonatoms, and a chlorine atom; or a cyclohexyl group optionally substitutedby an alkyl radical having 1 to 8 carbon atoms; and Z³ represents ahydrogen atom or a methyl radical.
 6. A liquid crystal composition asclaimed in claim 4, wherein ##STR268## same as defined in claim
 4. 7. Aliquid crystal composition as claimed in claim 4, wherein ##STR269##being the same as defined in claim
 4. 8. A liquid crystal composition asclaimed in claim 4, wherein R¹ represents an alkyl radical having 1 to 8carbon atoms or a 4-alkylcyclohexyl radical, R² represents ##STR270##wherein R⁶ represents an alkyl radical having 1 to 8 carbon atoms, or analkoxy radical having 1 to 8 carbon atoms, and R⁷ represents a hydrogenatom, or ##STR271## represents an alkyl radical having 1 to 3 carbonatoms, and R^(5') represents an alkyl radical having 1 to 8 carbonatoms, ##STR272## wherein R⁸ represents a hydrogen atom or a methylradical, or ##STR273## wherein R⁸ in the same as defined above,##STR274##
 9. The composition of claim 4 wherein m in Formula (IV) is 1,and R¹ is an alkyl radical having 1 to 8 carbon atoms.
 10. Thecomposition of claim 4 wherein m in Formula (IV) is 2, and R¹ is analkyl radical having 1 to 8 carbon atoms.
 11. The composition of claim 6wherein ##STR275## in Formula (IV) is ##STR276## wherein R⁸ is hydrogenor methyl.
 12. A liquid crystal composition as claimed in claim 6wherein ##STR277## in Formula (IV) is ##STR278## wherein R⁸ is hydrogenor methyl.
 13. A liquid crystal composition as claimed in claim 4wherein n in the general formula (IV) for the azo type compound is 1.